Turbo charging an engine allows the engine to provide power similar to that of a larger displacement engine while engine pumping work is maintained near the pumping work of a normally aspirated engine of similar displacement. Thus, turbo charging can extend the operating region of an engine. However, in severely downsized engines, the close coupling of the turbocharger and downstream exhaust components, such as catalysts, can present difficulties during high load engine operation. For example, the high exhaust temperatures of high load operation can damage the turbocharger and/or catalyst. To avoid degradation to the exhaust components, the engine may be operated with rich combustion in high load conditions to lower exhaust temperatures.
However, the inventors herein have identified potential issues with the above approach. For example, the rich operation may produce a significant amount of CO and HC emissions. Further, the lowered exhaust temperatures may result in these emissions going unconverted in the downstream catalysts.
Thus, in one example, some of the above issues may be at least partly addressed by an engine method comprising adjusting upstream exhaust air-fuel ratio to maintain a first emission control device at or below a maximum temperature, and when the upstream exhaust air-fuel ratio is below a threshold, injecting air into an exhaust passage between the first emission control device and a second emission control device to maintain downstream exhaust at a different, higher air-fuel ratio.
In this way, the temperature and air-fuel ratio of the downstream emission control device, which is further from the exhaust manifold and thus subject to lower exhaust temperatures than the upstream emission control device, may be adjusted independently of the upstream emission control device. By doing so, emissions that escape from the upstream emission control device unconverted may be converted in the downstream emission control device, without subjecting the upstream emission control device to high temperatures which may contribute to component degradation.
In one example, the temperature and air-fuel ratio of the downstream emission control device may be adjusted via the introduction of secondary air into an exhaust passage between the upstream emission control device and the downstream emission control device. The secondary air may include compressed intake air routed from the intake passage to the exhaust passage. Further, in some embodiments, the compressed intake air may be routed to the exhaust passage via an LP-EGR passage coupled the exhaust passage. The secondary air may be introduced based on feedback control to maintain a downstream air-fuel ratio at stoichiometry and/or maintain a temperature of the downstream emission control device within a threshold range.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.